Hotkey to Frame all Objects into viewport?
To frame all objects in one viewport do: Home or Ctrl + Home Both with the origin as the camera focal point
Or Shift + c
This will also bring the 3D cursor back to the origin.
Numpad . will frame the currently selected object or objects.
Numpad / will frame the currently selected object or objects and hide everything else, until you press it again.
You can change them here (to F):

This however will affect only the (single) viewport under the mouse cursor.
I imagine affecting multiple viewport at once will require a script, but i'm not sure.
EDIT:
If you want to experiment with a different hotkey layout you can always click Add Key-config Preset (the little &quot;+&quot; icon), to make a new preset and try some things without messing up you current key setup.